1. Field of the Invention
This invention relates generally to an amplifier, and is directed more particularly to an amplifier with a differential amplifier.
2. Description of the Prior Art
In the art there has been known such an amplifier which includes a differential amplifier formed of two transistors and a collector follower transistor connected directly to the output side of the differential amplifier. Such a prior art amplifier is shown in FIG. 1. As shown in FIG. 1, a differential amplifier A is formed of two transistors Q.sub.1 and Q.sub.2 whose emitter electrodes are connected together through an emitter resistor R.sub.2 to one terminal -V.sub.CC of a voltage source, and whose base electrodes are connected to input terminals T.sub.1 and T.sub.2, respectively. The collector electrode of the transistor Q.sub.2 is connected to the other terminal +V.sub.CC of the voltage source and the collector electrode of the transistor Q.sub.1 is connected through a resistor R.sub.1 to the terminal +V.sub.CC and also to the base electrode of another transistor Q.sub.3, whose emitter electrode is connected through a resistor R.sub.3 to the terminal +V.sub.CC and whose collector electrode is connected to an output terminal T.sub.3 and also to the terminal -V.sub.CC through a resistor R.sub.4.
With such a prior art amplifier, if the differential amplifier A is formed completely or ideally, even when an input signal with components which are the same in phase are supplied to the input terminals T.sub.1 and T.sub.2, respectively no signal components which are the same in phase are delivered to the output terminal T.sub.3.
However, due to the fact that the transistors Q.sub.1 and Q.sub.2 which form the differential amplifier A are not uniform in characteristics, when the same signal components in phase are fed to the input terminals T.sub.1 and T.sub.2, respectively, the same signal components in phase appear at the output terminal T.sub.3.
The total gain of the amplifier for the same signal components in phase or the common mode gain CMG is expressed as follows: EQU CMG = R.sub.1 /2R.sub.2. R.sub.4 /R.sub.3
in the above expression, the factor R.sub.1 /2R.sub.2 represents the common mode gain of the differential amplifier A itself, and the factor R.sub.4 /R.sub.3 the gain of the transistor Q.sub.3.
With the prior art amplifier, the resistor R.sub.2 is used as a constant current source which is made to have a high impedance to reduce the common mode gain.
However, in general the constant current source is formed of a transistor whose emitter electrode is grounded, so that it is impossible to select the impedance of the constant current source infinitely. Therefore, the prior art amplifier can not make its common mode gain zero.
If there is such a common mode gain in an amplifier, there occurs a defect that when the voltage source is made ON and OFF, and its source voltage is changed, noises are caused or the ripple in the voltage source is contained in the signal which deteriorates the S/N ratio.